39 U.S. Code § 1011 - Oath of office

Before entering upon their duties and before receiving any salary, all officers and employees of the Postal Service shall take and subscribe the following oath or affirmation:

“I, XXXXXXXX, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter.”

A person authorized to administer oaths by the laws of the United States, including section
2903 of title
5, or of a State or territory, or an officer, civil or military, holding a commission under the United States, or any officer or employee of the Postal Service designated by the Board may administer and certify the oath or affirmation.